Monolithic flexure pre-stressed ultrasonic horns

ABSTRACT

A monolithic ultrasonic horn where the horn, backing, and pre-stress structures are combined in a single monolithic piece is disclosed. Pre-stress is applied by external flexure structures. The provision of the external flexures has numerous advantages including the elimination of the need for a pre-stress bolt. The removal of the pre-stress bolt eliminates potential internal electric discharge points in the actuator. In addition, it reduces the chances of mechanical failure in the actuator stacks that result from the free surface in the hole of conventional ring stacks. In addition, the removal of the stress bolt and the corresponding reduction in the overall number of parts reduces the overall complexity of the resulting ultrasonic horn actuator and simplifies the ease of the design, fabrication and integration of the actuator of the present invention into other structures.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to and the benefit of U.S. provisionalpatent application Ser. No. 61/362,164 filed Jul. 7, 2010 and priorityto and the benefit of U.S. provisional patent application Ser. No.61/505,048 filed Jul. 6, 2011, each of which applications isincorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY FUNDED RESEARCH OR DEVELOPMENT

The invention described herein was made in the performance of work undera NASA contract, and is subject to the provisions of Public Law 96-517(35 USC 202) in which the Contractor has elected to retain title.

THE NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENT

Not Applicable

INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC

Not Applicable

FIELD OF THE INVENTION

The invention relates to ultrasonic horns in general and particularly toultrasonic horns that employ pre-stressed actuator elements.

BACKGROUND OF THE INVENTION

A variety of industrial applications exist where power ultrasonicactuators, such as ultrasonic horns, are used to produce large amplitudevibrations. These applications include medical/surgical, automotive,food preparation, and textile cutting applications, as well as use infabrication industries and material joining. Ultrasonic actuators areattractive for their ability to generate precision high strokes, torquesand forces while operating under relatively harsh conditions, such astemperatures in the range of single digit Kelvin to 1273 Kelvin. Detailsrelated to a variety of applications can be found in the followingreferences: A. Shoh, “Industrial Applications of Ultrasound—A review 1.High Power Ultrasound”, IEEE Trans on Sonics and Ultrasonics, SU-22, 2,pp. 60-71, 1975; L. Parrini, “New Methodology For The Design Of AdvancedUltrasonic Transducers For Welding Devices”, Proceedings of the IEEEInternational Ultrasonics Symposium, pp. 699-714, 2000; W. W. Cimino, L.J. Bond, “Physics of Ultrasonic Surgery Using Tissue Fragmentation,Proceedings of the IEEE International Ultrasonics Symposium, pp.1597-1600, 1995; K. F. Graf, Process Applications of Power Ultrasonics—AReview”, Proceedings of the IEEE International Ultrasonics Symposium,pp. 628-641, 1974.

Known in the prior art is Stegelmann, U.S. Pat. No. 7,754,141, issuedJul. 13, 2010, which is said to disclose in one aspect an ultrasonichorn for transporting ultrasonic energy to an operating locationdefining a radial direction and an axial direction. The ultrasonic hornincludes a horn member and an energy transfer surface disposed on thehorn member. The ultrasonic horn also includes an axle member joined tothe horn member where the axle member is provided by a first material.The ultrasonic horn further includes an isolation member integrallyjoined to the axle member and adapted for mounting the ultrasonic hornat a work location where at least a portion of the isolation member isprovided by a second material.

Stegelmann further discloses that the first material and the secondmaterial can exhibit different properties for transporting ultrasonicenergy. In a particular aspect, the axial isolation submember can beacoustically decoupled from the axle member. According to Stegelmann,configuring the isolation member in this manner provides severaladvantages. For instance, the isolation member will suitably transfer areduced amount of vibration to the coupler. Accordingly, this canadvantageously decrease noise associated with the ultrasonic horn,improve performance due to the lower vibration and improve the mountingof the horn. Moreover, equipment life of both the horn and the couplercan be improved.

As is known in the prior art, the application of pre-stress orpreloading can be used to maintain the integrity of the piezoelectricmaterial in an actuator. Known in the prior art is Chambers et al., U.S.Pat. No. 7,327,637, issued Feb. 5, 2008 (herein after referred to as theChambers et al. '637 patent) and an article entitled “Characterizationof piezoelectrically induced actuation of Ni—Mn—Ga singe crystals” byChambers et al. (Smart Structures and Materials 2005: Active Materials:Behavior and Mechanics, edited by William D. Armstrong, Proceedings ofSPIE Vol. 5761 (SPIE, Bellingham, Wash., 2005), pages 478-489) (hereinafter referred to as the Chambers et al. Induced Actuation article).These references are said to disclose in part the actuation of magneticmaterials using stress waves. In particular the Chambers et al. '637patent discloses an acoustic actuator, including an acoustic stress wavegenerator and an actuation material operatively positioned relative tothe acoustic stress wave generator for the delivery of acoustic stresswaves from the generator to actuation material. The application of apre-stress is shown and discussed with respect to FIG. 6 of the Chamberset al. '637 patent and FIG. 1 of the Chambers et al. Induced Actuationarticle (herein reproduced as FIG. 1). Also disclosed is that the clampsurrounding the piezoelectric stack (the moonie clamp) holds andpositions the stack, without the use of a bonding agent. It alsoprovides a prestress on the stack, which improves its performance. TheMoonie clamp can be driven in the transverse direction. In this type ofactuator, when the stack expands the work surface contracts and when thestack contracts the work surface expands. In the structures disclosed inthe Chambers et al. '637 patent and Induced Actuation article, theclamping function but not the transverse amplification is employed.Additional details regarding the Moonie clamp can be found in thearticles entitled “Metal-Ceramic Composite Transducer, the ‘Moonie’” byOnitsuka et al. (Journal of Intelligent Material Systems and Structures,July 1995, Vol. 6 No. 4, pages 447-455), “Flextensional ‘Moonie’Actuator” by R. E. Newnham et al. (1993 Ultrasonics Symposium, pages509-513, IEEE, 1051-0117/93/0000-0509), and “Hallow PiezoelectricComposites” by J. F. Fernandez et al. (Sensors and Actuators A, 51,pages 183-192 (1996)).

The structures in the Chambers et al. '637 patent and Induced Actuationarticle are directed to low power and low frequency applications,principally below 100 Hz, and are well suited for micropositioningapplications, as the patent states. If stress levels in addition tothose provided by the piezoelectric stack are required, the Chambers etal. '637 patent discloses that a separate acoustic horn can be placedbetween the piezoelectric material and the actuation material.

In generating stress waves at low frequencies, the devices disclosed inthe Chambers et al. '637 patent and Induced Actuation article are notdriven at resonance. In fact resonances are described by the Chambers etal. '637 patent and Induced Actuation article as poorly understoodunwanted artifacts. When discussing the measured propagated stress wavein the FSMA actuation material resulting from the control voltage pulseplotted in FIG. 7A of the Chambers et al. '637 patent, the patentdiscloses that it is found that this stress wave is not ideal, reachingnearly the same tensile stress as compressive stress. The oscillationsalong the peak of the stress wave are disclosed to be due to the lengthof the input control pulse relative to the period of the resonance ofthe piezoelectric stack. The Chambers et al. '637 patent indicates thata sound wave can travel from one end of the stack to the otherapproximately 7 times in 50 μs, based on the speed of sound calculatedfrom stack properties, resulting in the 7 small peaks seen along themajor peak of the wave. In discussing the effect of repetition rate onpeak-to-peak output stress on a full actuation cycle, the Chambers etal. Induced Actuation article states that the data shows a peak at 70Hz, along with several other local minima and maxima. They disclose thatthey believe these features are associated with acoustic resonances inthe actuator itself, or the spring load system. However, they state thatthe details of the resonances are not understood well.

Pre-stressing the material becomes especially beneficial when thepiezoelectric material is driven at high power. Barillot et al, U.S.Pat. No. 6,927,528, issued Aug. 9, 2005, is said to disclose in part thedamping capacities of a piezoelectric actuator and its resistance todynamic external stresses. One embodiment shown in FIG. 7 in Barillot,herein reproduced as FIG. 2, discloses that to increase the capacity ofthe actuator to resist higher external stresses, the preloading of thepiezoelectric components can also be increased. It is further disclosedthat this preloading is normally performed by the actuator shell 31, butits value is limited in practice by the elastic limit of the material ofthe shell 31. Barillot further states that it may therefore beadvantageous to add an additional preloading device 35 arranged inparallel to the large axis 16 of the actuator so as to increase thecapacity of the actuator to resist external stresses. To use such asystem, the use of an extruded shell 31 is indicated as beingparticularly useful. Barillot states that two springs can be connectedto the shell 31 along its large axis to increase the preloading on thepiezoactive components.

Another approach also known in the prior art to pre-stressingpiezoelectric material in high power ultrasonic actuators is the use ofa stress bolt as shown in FIG. 3A. Typically these actuators areassembled with a horn, piezoelectric rings, backing and a stress bolt.The ring elements are connected electrically in parallel and placedbetween the horn and the backing ring. The pre-stress bolt is insertedthrough a backing ring and the piezoelectric rings and screwed into thehorn until a desired pre-stress level, such as greater than 20 MPa, isreached. Another example of pre-stressing using a bolt is disclosed inthe international application identified by the World InternationalPatent Organization International Publication Number WO 03/026810 A1,which example is reproduced as FIG. 3B. This document discloses in partthat a piezoelectric element 1 comprising two piezoelectric ceramicrings and thin electrodes is held in compression by a pre-load between asteel rear mass 2 and the horn 3. The pre-load is provided by apultruded glass fibre tube 4 under compression in a sliding fit with ahigh-tensile cap-head bolt 5. A washer 6 is inserted between the head ofthe bolt and the rear face of the pultruded tube. The cap head bolt isscrewed into the transducer horn adjacent to the piezoelectric ceramicat 7, securing the arrangement. Tightening the cap-head bolt 5 forcesthe pultruded tube 4 to remain under compression. The washer 6 ensuresthat the torsional force on the tube and consequently on thepiezoelectric ceramic is kept to a minimum. Also known in the prior areis the use of an insulating bolt to address the issue of internalelectrical discharge or to eliminate the need for an insulating coveringover a traditional metallic bolt. For example the insulating bolt can bemade of E glass which has a compliance that is twice that of steel andnearly as strong.

A number of problems in ultrasonic horns that are constructed with apre-stress bolt have been observed. One of these problems is thatultrasonic horns with pre-stress bolts are susceptible to electricaldischarge and mechanical failure. In addition typically ultrasonic hornsinclude numerous components. A high component count leads to the resultthat their design, manufacture, assembly and integration into otherstructures can be complicated and costly. It can also be expensive andtime consuming to make an actuator element having a hole defined thereinthat is designed to allow the pre-stress bolt to pass through. The holedefined in an actuator element can also be a “stress raiser” which canlead to mechanical failure of the actuator.

There is a need for an ultrasonic horn that addresses the issues offabrication and assembly complexity and as well as performance failureissues.

SUMMARY OF THE INVENTION

According to one aspect, the invention features an ultrasonic horn for ahigh power actuator. The ultrasonic horn comprises a monolithicpre-stress portion constructed from a first material, the monolithicpre-stress portion having a first interior surface and a second interiorsurface defined therein, the first interior surface and the secondinterior surface configured to provide a pre-stress cavity for a highpower actuator material; and a horn portion connected to the monolithicpre-stress portion, the horn portion is configured to be driven at aresonance frequency.

In one embodiment, the first interior surface and the second interiorsurface are monolithically connected by at least one flexure structure.In another embodiment the at least one flexure structure is configuredto apply a specified pre-stress having a pre-stress value to an actuatormaterial inserted in the pre-stress cavity. In an additional embodiment,the pre-stress value varies less than one part in a hundred as atemperature varies over 100 degrees Celsius. In a further embodiment,the at least one flexure has a first stiffness value and the actuatormaterial has a second stiffness value, the first stiffness value beingat least a factor of 10 less than the second stiffness value. In yetanother embodiment, a ratio between the first stiffness value and thesecond stiffness value is configured to reduce a mechanical creep. Inyet an additional embodiment, a ratio between the first stiffness valueand the second stiffness value is configured to increase a couplingvalue. In another embodiment, the pre-stress value varies less than onepart in 20 as the first material deforms by one part in 20. In stillanother embodiment, the specified pre-stress is directed along a firstaxis and the horn portion amplifies displacement along the first axis.In another embodiment, the actuator material is a piezoelectricmaterial. In still an additional embodiment, the actuator material has aset of exterior dimensions and lacks a through hole. In still a furtherembodiment, the first material comprises titanium. In yet still anotherembodiment, the resonance frequency is at least five thousand Hertz. Inyet still an additional embodiment, the ultrasonic horn actuator isdriven by a power of at least 20 Watts. In yet still a furtherembodiment, the ultrasonic horn actuator is provided as a component in amedical device. In another embodiment, the ultrasonic horn actuator isattached to a support structure at a node of a resonant mode of theresonant frequency.

According to another aspect, the invention relates to a method ofmanufacturing an ultrasonic horn for a high power actuator. The methodcomprises the step of forming a monolithic pre-stress portionconstructed from a first material, the monolithic pre-stress portionhaving a first interior surface and a second interior surface definedtherein, the first interior surface and the second interior surfaceconnected by at least one flexure, the first interior surface, thesecond interior surface and the at least one flexure configured toprovide a pre-stress cavity for a high power actuator material; andforming a horn portion connected to the monolithic pre-stress portion,the horn portion configured to be driven at a resonance frequency.

In another embodiment, the step of forming a monolithic pre-stressportion comprises forming the monolithic pre-stress portion usingelectron beam melting. In an additional embodiment, the step of forminga monolithic pre-stress portion comprises forming the monolithicpre-stress portion using rapid prototyping. In a further embodiment, thestep of forming a monolithic pre-stress portion comprises forming themonolithic pre-stress portion by precision machining.

The foregoing and other objects, aspects, features, and advantages ofthe invention will become more apparent from the following descriptionand from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and features of the invention can be better understood withreference to the drawings described below, and the claims. The drawingsare not necessarily to scale, emphasis instead generally being placedupon illustrating the principles of the invention. In the drawings, likenumerals are used to indicate like parts throughout the various views.

FIG. 1 is a drawing of a clamp used to apply pre-stress as known in theprior art;

FIG. 2 is a drawing of a device used to apply pre-stress as known in theprior art;

FIG. 3A and FIG. 3B are drawings of devices known in the prior art thatare used to apply pre-stress using stress bolts;

FIG. 4 illustrates an exemplary embodiment of a monolithic pre-stresshorn actuator according to the invention;

FIG. 5 illustrates an exemplary embodiment of a monolithic pre-stresshorn actuator including an actuator material according to the invention;

FIG. 6 is a plot of a static Von Mises stress profile for a given gapseparation for one embodiment of the invention;

FIG. 7 shows an illustrative schematic of the EBM manufacturing processused in one embodiment to manufacture a monolithic pre-stress hornactuator according to the invention;

FIG. 8A shows an image of a rapid prototype and EBM manufacturedTitanium ultrasonic pre-stress horn actuator according to the invention;

FIG. 8B shows two assembled titanium alloy (Ti-6Al-4V) ultrasonic hornprototypes manufactured using EBM with piezoelectric stacks insertedaccording to the invention;

FIG. 9 shows the impedance spectrum for one embodiment of the invention;

FIG. 10 shows a portion of a support rig that was used to assemble oneof the embodiments according to the invention;

FIG. 11A shows the admittance spectra for one embodiment of theinvention;

FIG. 11B shows the phase spectrum for one embodiment of the invention;

FIG. 12A is an image of a device according to the principles of theinvention with a testing apparatus; and

FIG. 12B is a CAD rendering of an illustrative example of a Barth motorimplemented according to the principles of the invention.

DETAILED DESCRIPTION

One embodiment of the present invention describes a monolithicultrasonic horn where the horn, backing, and pre-stress structures arecombined in a single monolithic piece. According to one embodiment ofthe invention, pre-stress is applied by external flexure structures.These flexures are designed to produce the appropriate stress whenassembled with an actuation material, such as a piezoelectric materialor an electrostrictive material.

The provision of the external flexures has numerous advantages includingthe elimination of the need for a pre-stress bolt. The elimination ofthe pre-stress bolt eliminates potential internal electric dischargepoints in the actuator. In addition, it reduces the chances ofmechanical failures in the actuator material that result from the freesurface in the hole of conventional ring stacks. In addition theelimination of the stress bolt and the corresponding reduction in theoverall number of parts reduces the overall complexity of the resultingultrasonic horn actuator and simplifies the ease of the design,fabrication and integration of an actuator into other structures.

Another advantage of this embodiment is that the actuator volume is notreduced to accommodate the stress bolt. This allows for an increase inthe total energy density. In addition, the pre-stress is not limited bybolt diameter. Further, production of piezoelectric plates without ahole has a higher yield and is thus less expensive. Also, by reducingthe stiffness and increasing the displacement of the external flexureone can increase the electromechanical coupling of the actuator. Anadditional benefit is the increased thermal preload and mechanical creepstability due to the increase by about an order of magnitude or more ofthe compliance of the spring system. According to one embodiment of theinvention, the actuator material is at least a factor of ten timesstiffer than the flexure structure. According to another embodiment, thepre-stress value varies less than one part in 20 as the monolithicpre-stressed actuator material deforms by one part in 20. According toanother embodiment, the pre-stress value varies less than one part in ahundred as the temperature varies over one hundred degrees Celsius.Traditional systems that use pre-stress bolts that have a higherstiffness are more prone to variations in pre-stress due to temperaturevariation and mechanical creep. It is also noted that at high pre-stressand high frequency one does not have to resort to exotic alloys toaccommodate the stress and fatigue in the stress bolt.

According to the principles of the present invention, the use offlexures and rapid prototyping can effectively be applied to themanufacture of power ultrasonic horns. The invention contemplatesmanufacturing the monolithic ultrasonic pre-stress horn actuators from asingle material or plate, in an array in a plate, or in a 3D structure.The invention also enables the integration of high power horns into 2Dor 3D structures. In one embodiment of the present invention,fabrication of the horns was achieved with electron beam melting (EBM).In other embodiments, other rapid prototyping techniques in addition toprecision machining are employed. In additional embodiments, fabricationis achieved using such low cost high production techniques as investmentcasting. The approach of the present invention in using flexures canalso be scaled to miniaturized horns for other specialized applicationslike camera motors and miniature zoom lenses. Precision machining can beemployed to produce an ultrasonic horn having precisely controlleddimensions from a rapidly produced monolithic pre-form.

Depending on the application, as known to one of skill in the art,ultrasonic horns can be produced in a variety of configurationsincluding constant, linear, exponential and stepped cross sections.These names refer to the degree in which the area changes along thelength of the horn from the base to the tip. A magnification in thestrain occurs in the stepped horn that in general is a function of theratio of diameters. In addition, the device is generally driven atresonance to further amplify the strain. The resonance amplification isdetermined by the mechanical Q (attenuation) of the horn material andradiation damping. The horn length primarily determines the resonancefrequency. For example for a 22 kHz resonance frequency, a stepped hornof titanium has a length of approximately 8 cm. Other more complicatedhorn structures are described in the following references: U.S. Pat. No.6,863,136 entitled “Smart-ultrasonic/sonic driller/corer” issued Mar. 8,2005 to Yoseph Bar-Cohen, Stewart Sherrit, Benjamin Dolgin, ThomasPeterson, Dharmendra Pal, Jason Kroh, and Ron Krahe; S. Sherrit, M.Badescu, X. Bao, Y. Bar-Cohen, and Z. Chang, “Novel Horns for PowerUltrasonics,” Proceedings of the IEEE International UltrasonicsSymposium, UFFC, Montreal, Canada, Aug. 24-27, 2004; S. Sherrit, B. P.Dolgin, Y. Bar-Cohen, D. Pal, J. Kroh, T. Peterson “Modeling of Hornsfor Sonic/Ultrasonic Applications”, Proceedings of the IEEE UltrasonicsSymposium, pp. 647-651, Lake Tahoe, October 1999; S. Sherrit, S. A.Askins, M. Gradziol, B. P. Dolgin, X. Bao, Z. Chang, and Y. Bar-Cohen,“Novel Horn Designs for Ultrasonic/Sonic Cleaning Welding, Soldering,Cutting and Drilling,” Paper 4701-34, Proceedings of the SPIE SmartStructures and Materials Symposium, San Diego, Calif., Mar. 17-19, 2002;and X. Bao, Y. Bar-Cohen, Z. Chang, B. P. Dolgin, S. Sherrit, D. S. Pal,S. Du, and T. Peterson, “Modeling and Computer Simulation ofUltrasonic/Sonic Driller/Corer (USDC)”, IEEE Transactions onUltrasonics, Ferroelectrics, and Frequency Control, vol. 50, no. 9, pp.1147-1160, September 2003.

According to one embodiment of the invention, the appropriate flexuregeometry for one particular configuration was determined by modeling themonolithic horn by finite element analysis using SolidWorks (availablefrom Dassault Systemes SolidWorks Corp., 300 Baker Avenue, Concord,Mass. 01742) and COSMOS analysis software (available from DassaultSystemes SolidWorks Corp., 300 Baker Avenue, Concord, Mass. 01742).

A solid model of an embodiment of a monolithic pre-stress horn actuator10 having these flexures is shown in FIG. 4. The monolithic pre-stresshorn actuator 10 has a horn tip 14, flexures 18, an actuator materialcavity 22 defined by cavity interior surfaces 26. In one embodiment, themonolithic pre-stress horn actuator 10 is held at a nodal plane that isdesigned to be between the flexures 18 and the horn tip 14. According tothis embodiment, affixing the monolithic pre-stress horn actuator 10 ata nodal plane reduces the energy from the resonant mode that is lost toa holding or support structure. In FIG. 5 is shown a horn with flexuresincluding an actuator material 30, such as a piezoelectric stack,inserted in the cavity 22 that is formed by the interior surfaces 26 ofthe two portions of the horn that are joined by the flexures 18. Astatic Von Mises stress profile for a given gap separation for thisembodiment is shown in FIG. 6. The stresses shown in FIG. 6 areprincipally larger within the flexures.

In order to design a flexure which acts like a spring by remaining inthe material's elastic region, the flexure was designed in oneembodiment to operate at stresses lower than the material's yieldstrength. According to one embodiment, to ensure that a piezoelectrictransducer (PZT) never experienced tensile stress under maximal drivevoltages, the piezoelectric horn was design to achieve a compressivepressure of 20 MPa on the inner surfaces of the piezoelectric materialcavity. This is the compressive pressure that would be exerted on a PZTtransducer inserted in the cavity. In one embodiment, a 25.4 mm diameterPZT cylinder had an area of 5.1 cm². In order to experience 20 MPa ofstress, the PZT required a compressive force of 10.1 kN. In a currentpreferred embodiment, the pre-stress is exerted along the centralsymmetry axis of the monolithic pre-stress horn actuator. In oneembodiment, the displacement amplification of the horn actuator is alongthe same central symmetry axis.

The stress calculations determined that the deformation of the PZTactuator was negligible since the ceramic material has a high stiffnesscompared to the flexure. The SolidWorks design was modeled and tested inCOSMOS and the results were compared with IDEAS values, IDEAS is afinite element analysis software. I-DEAS (Integrated Design EngineeringAnalysis Software) was developed by UGS, which was acquired by SiemensPLM software having an office at 5800 Granite Parkway, Suite 600, Plano,Tex. 75024. After choosing the manufacturing technique and material, itwas possible to determine the proper dimensions of the flexure and itsgap to allow for a 20 MPa pre-stress when the flexure was pulled toproduce a 0.13 mm increase in distance from the nominal separation ofthe 9.2 mm gap in order to accommodate the 9.33 mm PZT stack thickness.The titanium flexure could be opened and the PZT placed in position witha factor of safety of approximately 1.89. The highest stress occurred onthe inside of the flexure at the inside edges. This surface wasthickened and given a large fillet to reduce the stress concentration.Another consideration was the risk of fatigue failure, since theactuator is operating at 30 kHz enduring thousands of cycles eachsecond. In one embodiment the material contemplated is Ti-6Al-4V. Theprinciples of the invention are not limited to a particular material andthe various embodiments contemplate the use of the materials includingbut not limited to aluminum, steel, titanium and any other alloymaterial that can be casted. In one embodiment, multiple materials suchas different alloys are used in different parts of the structure to, forexample, provide for different stiffness values such as a softerstiffness value for the flexures.

In order to investigate rapid prototyping in the manufacture of hornsaccording to one embodiment of the invention, the monolithic horns weremanufactured in Ti-6Al-4V using electron beam melting/manufacturing(EBM). A schematic of the EBM manufacturing process is shown in FIG. 7.In the EBM process, fully dense metal parts are built up layer-by-layerby melting metal powder using a powerful electron beam. Each layer ismelted to substantially the geometry defined by a 3D CAD model. The EBMtechnology allows for high energy to be used providing high meltingcapacity and high productivity. Parts are built in vacuum at elevatedtemperatures resulting in stress-relieved parts with material propertiesbetter than cast and comparable to wrought material. Referring to FIG.7, the EBM process starts with electrons being emitted from a filamentwhich is heated to greater than 2500° C. The electrons are acceleratedthrough the anode to half the speed of light. A magnetic field lensbrings the beam into focus. Another magnetic field controls the movementof the beam. When the electrons hit the powder, kinetic energy istransformed to heat. The heat melts the metal powder. For each layer ofpowder, the electron beam first scans the powder bed to maintain acertain elevated temperature, specific for each of different alloys.Thereafter the electron beam melts the contours of the part and finallythe bulk. Two companies involved in EBM production are CalRAM Inc.having a place of business at 2380 Shasta Way, Suite B, Simi Valley,Calif. 93065 USA and Arcam AB having a place of business at KrokslättsFabriker 27A, SE-431 37, Mölndal, Sweden. Additional details regardingthe EBM process can be found in materials located on the internet pagesmaintained by these companies.

In the EBM process the titanium parts can be made to an accuracy ofabout 0.4 mm with strengths comparable to cast and wrought materials.The part quality is such that they are now used in both the aerospaceand medical implant fields. The EBM manufacturing approach is useful forsmall production runs. According to one embodiment for larger productionand cheaper cost per part, use of the investment casting tree approachis contemplated as part of the invention. In this approach it is alsopossible to co-cast, for example, stainless steel and titanium.

FIG. 8A shows an image of a rapid prototype and EBM manufacturedtitanium alloy ultrasonic pre-stress horn actuator. A final millingfinished the surfaces that contact the piezoelectric stack. FIG. 8Bshows two assembled titanium alloy (Ti-6Al-4V) ultrasonic hornprototypes manufactured using EBM with piezoelectric stacks. The EBMmanufactured horn critical surfaces were finished using a standard milland lathe. The United States quarter dollar coin provides a referencescale.

According to one embodiment, the piezoelectric stacks were purchasedfrom Piezomechanik Gmbh. In this embodiment, the bi-polar stacks werenominally 25.4 mm OD and 9.33 mm thick. The impedance spectrum of thefirst length extensional mode for these stacks is shown in FIG. 9. Theimpedance spectra of the assembled horns shown in FIG. 8B were measuredon a HP 4294a Impedance analyzer. An analysis of the small signalresonance data of the bare stack gave an effective piezoelectricconstant of 480 pC/N for the material and a capacitance of 261 nF. Thecoupling was determined to be k₃₃=0.56 and the elastic compliance atconstant field in the 33 direction was 5.4×10-11 m²/N. The mechanical Qwas in the 40-80 range. Those skilled in the art will recognize thatpiezoelectric material from another source and having other dimensionscan be used without departing from the inventive concepts disclosedherein.

In order to open the flexure to install the stack, a support structurewas designed to move the flexures so as to pull the first interiorsurface and the second interior surface apart sufficiently to allow fora piezoelectric stack coated with 3M 2216 epoxy to be inserted. Part ofthe support rig that was used to open the flexures is shown in FIG. 10.The horn tip is clamped between compression plates in a vice. The otherend of the ultrasonic pre-stress horn has a blind threaded hole providedtherein. A bolt is used to pull the threaded end of the ultrasonicpre-stress horn away from the horn base. An aluminum block is fixed tothe vice surface to support the other end of the bolt. When the flexureswere released, the voltage on a 10 microfarad capacitor connected inparallel with the piezoelectric stack was monitored to provide a valuefor the applied pre-stress. Upon releasing the flexure, the chargegenerated on the piezoelectric actuator indicated a pre-stress greaterthan 25 MPa. In one embodiment, if the pre-stress is less than a targetvalue, then shims can be inserted to raise the pre-stress level to thetarget value when the actuator material is inserted in the pre-stresscavity. The necessary thickness of the shims can be calculated or can bedetermined by testing, based on a measurement of the parallel capacitor.In another embodiment, if the pre-stress level of the monolithicpre-stress ultrasonic horn is greater than a target value, someadditional material can be removed from one or more of the interiorsurfaces of the pre-stress actuator material cavity so as to achieve thetarget pre-stress level. The target pre-stress levels will varydepending on the application for the monolithic pre-stress ultrasonichorn and on the actuator material used.

The admittance spectra of the assembled horns shown in FIG. 8B are shownin FIG. 11A and the phase spectra are shown in FIG. 11B. From this data,the frequencies were found to be 28.5-29.3 kHz and the coupling wasmeasured to be in the range k=0.20-0.21, which is an improvement over astandard horn with the same dimensions with internal stress-bolt.Modeling of horns with more compliant springs predict coupling of k=0.3and higher. This is significantly better than similar horns producedwith a stress bolt. The mechanical Q's of the horn were measured to be106 and 115 which is larger than the Q of the bare piezoelectric stacks(Q≈40-80). These results for the flexure stack horns suggest thatembodiments of high power ultrasonic horns can be produced that havecoupling coefficients that are equal to or greater than standard hornsmanufactured with a stress bolt. In addition these horn structures havethe advantage that the piezoelectric volume for a given length can beincreased. Also, the removal of the stress bolt removes a potentialdischarge point internal to the piezoelectric and removes a free surfacefor cracks to initiate on. In other embodiments, the resonant frequencyof the monolithic pre-stress ultrasonic horn can be in the range of 5kHz to 200 kHz. In other various embodiments, the power levels appliedto a high power horn actuator of the present invention are up tohundreds of Watts. In some embodiments, the principles of the inventionare applied to low power applications such as camera motors andminiature zoom lenses and the like. In these embodiments the powerlevels can be in the milliwatt or lower range. Additional low powerembodiments include a wide variety of applications in which a mechanismcapable of generating very small oscillations is required.

The ultrasonic pre-stress monolithic horns of one embodiment of theinvention shown in FIG. 8B were generated using the EBM process.However, if large quantities were required, the ultrasonic pre-stressmonolithic horns of the present invention could also be manufacturedusing such production run techniques as investment casting,water-jetting, and EDM. In addition one could also include sacrificialfeatures like a larger diameter horn tip or a threaded hole in thebacking to aid in the assembly.

The use of flexures as opposed to stress bolts to generate thepre-stressing provides additional advantages of the present invention.Some of these advantages are related to the flexures having a lowerspring constant than a stress bolt. One benefit of this is thermalpreload stability in which operation over a larger temperature range, upto the Curie Temperature of the piezoelectric material that is used, isenabled. In addition by reducing the spring constant and increasing thedisplacement, less energy is required in the spring material atresonance and there is less potential for dynamic stress which inducesfatigue. In the “Modeling of Horns for Sonic/Ultrasonic Applications”paper cited above, it was determined that a stepped horn with a stressbolt had a coupling of k=0.18. A re-analysis of the same data indicatedthat if the stress bolt in the model was removed, the coupling was foundto increase to k=0.34. The replacement of the stress bolt with theflexures, therefore, corresponds to a replacement by a very soft springwith a stiffness of an order of magnitude or more smaller. The softerspring improves the coupling.

FIG. 12A and FIG. 12B show one of the many applications for anultrasonic horn of the present invention. In the embodiments shown inFIG. 12A and FIG. 12B, the monolithic pre-stress ultrasonic horns weredesigned to be implemented in a Barth motor. FIG. 12A is an image of thedevice with a testing apparatus. FIG. 12B is a CAD rendering of anillustrative example of a Barth motor produced by mounting a flexureultrasonic horn of the present invention against a rotor. Testing of themotor with a horn of the present invention demonstrated a rotor speed of15 RPM and a torque of approximately 0.3 N-m. The torque was determinedby hanging a mass around the shaft, then measuring the constant rate atwhich the mass was lifted against gravity.

Additional disclosure related to flexure ultrasonic horns can be foundin Monolithic Rapid Prototype Flexured Ultrasonic Horns by S. Sherrit,X. Bao, M. Badescu, Y. Bar-Cohen, and P. Allen published in IEEEInternational Ultrasonics Symposium, San Diego, October 2010.

Other applications for the use of a monolithic pre-stress horn actuatorof the present invention include but are not limited to structurallyintegrated motors, ultrasonic drilling including rotary hammering drillsdriven by a single piezoelectric stack, ultrasonic rock crushing,ultrasonic levitation, ultrasonic driller/corer (USDC), industrialapplications such as cutting and welding, medical applications includingsurgical tools, lithotripsy, knifes, and drills, space applicationsincluding corers, drills, abrasion tools and powder samplers, andtesting equipment such as wearing testing and fatigue testing.

While several of the embodiments of the invention mentioned abovediscussed the use of piezoelectric material, the invention alsocontemplates the use of other actuation material. These materialsinclude but are not limited to electrostrictive materials,magnetorestrictive materials, and thermal and ferromagnetic shape memoryalloys.

In piezoelectric materials, the strain S is proportional to electricfield E, S=d·E where d is the piezoelectric constant. Inelectrostrictive materials, the strain S is proportional to E².Electrostriction applies to all crystal symmetries, while thepiezoelectric effect only applies to the 20 piezoelectric point groups.In addition, unlike piezoelectricity, electrostriction cannot bereversed; that is, deformation will not induce an electric field.Electrostrictive materials can be driven like a tuned piezoelectric. IfS=Q·E² then by applying a bias field E and an ac field dE, a tunedpiezoelectric can be achieved, with dS=(2·Q·E)dE. Many piezoelectric andelectrostrictive materials are ceramic (PZT, PZN-PT). Ceramics generallyexhibit tensile strength that is a fraction of their compressivestrength. If driven at high fields and at high power (high fields andhigh frequency), they may go in tension and display reduced life due tocrack generation if the material is not preloaded. As discussed above byapplying a DC compressive preload, the power that may be applied to theactuator without fear of destruction is increased.

DEFINITIONS

Unless otherwise explicitly recited herein, any reference to anelectronic signal or an electromagnetic signal (or their equivalents) isto be understood as referring to a non-volatile electronic signal or anon-volatile electromagnetic signal.

THEORETICAL DISCUSSION

Although the theoretical description given herein is thought to becorrect, the operation of the devices described and claimed herein doesnot depend upon the accuracy or validity of the theoretical description.That is, later theoretical developments that may explain the observedresults on a basis different from the theory presented herein will notdetract from the inventions described herein.

Any patent, patent application, or publication identified in thespecification is hereby incorporated by reference herein in itsentirety. Any material, or portion thereof, that is said to beincorporated by reference herein, but which conflicts with existingdefinitions, statements, or other disclosure material explicitly setforth herein is only incorporated to the extent that no conflict arisesbetween that incorporated material and the present disclosure material.In the event of a conflict, the conflict is to be resolved in favor ofthe present disclosure as the preferred disclosure.

While the present invention has been particularly shown and describedwith reference to the preferred mode as illustrated in the drawing, itwill be understood by one skilled in the art that various changes indetail may be affected therein without departing from the spirit andscope of the invention as defined by the claims.

What is claimed is:
 1. An ultrasonic horn for a high power actuator,comprising: a monolithic pre-stress portion constructed from a firstmaterial, said monolithic pre-stress portion having a first interiorsurface and a second interior surface defined therein, said firstinterior surface and said second interior surface configured to providea pre-stress cavity for a high power actuator material, said specifiedpre-stress is directed along a first axis, said first interior surfaceand said second interior surface monolithically connected by at leastone flexure structure having a curved shape and configured to apply aspecified pre-stress value to an actuator material inserted in saidpre-stress cavity, said at least one flexure structure is configured toapply a specified pre-stress having a pre-stress value that varies lessthan one part in a hundred as a temperature varies over 100 degreesCelsius to an actuator material inserted in said pre-stress cavity; anda horn portion connected to said monolithic pre-stress portion, saidhorn portion is configured to be driven at a resonance frequency, andsaid horn portion is configured to amplify displacement along said firstaxis.
 2. The ultrasonic horn of claim 1 wherein said at least oneflexure has a first stiffness value and said actuator material has asecond stiffness value, said first stiffness value being at least afactor of 10 less than said second stiffness value.
 3. The ultrasonichorn of claim 1 wherein said pre-stress value varies less than one partin 20 as said first material deforms by one part in
 20. 4. Theultrasonic horn of claim 1 wherein said actuator material is apiezoelectric material.
 5. The ultrasonic horn of claim 1 wherein saidactuator material has a set of exterior dimensions and lacks a throughhole.
 6. The ultrasonic horn of claim 1 wherein said first materialcomprises titanium.
 7. The ultrasonic horn of claim 1 wherein saidresonance frequency is at least five thousand Hertz.
 8. The ultrasonichorn of claim 1 wherein said ultrasonic horn actuator is driven by apower of at least 20 Watts.
 9. The ultrasonic horn of claim 1 whereinsaid ultrasonic horn actuator is provided as a component in a medicaldevice.
 10. The ultrasonic horn of claim 1 wherein said ultrasonic hornactuator is attached to a support structure at a node of a resonant modeof the resonant frequency.
 11. The ultrasonic horn of claim 1 whereinsaid resonance frequency is primarily determined by a length of saidhorn.
 12. An ultrasonic horn for a high power actuator, comprising: amonolithic pre-stress portion constructed from a first material, saidmonolithic pre-stress portion having a first interior surface and asecond interior surface defined therein, said first interior surface andsaid second interior surface configured to provide a pre-stress cavityfor a high power actuator material, said specified pre-stress isdirected along a first axis, said first interior surface and said secondinterior surface monolithically connected by at least one flexurestructure having a curved shape and configured to apply a specifiedpre-stress value to an actuator material inserted in said pre-stresscavity, said at least one flexure structure is configured to apply aspecified pre-stress to an actuator material inserted in said pre-stresscavity, said at least one flexure having a first stiffness value andsaid actuator material having a second stiffness value, said firststiffness value being at least a factor of 10 less than said secondstiffness value; and a horn portion connected to said monolithicpre-stress portion, said horn portion is configured to be driven at aresonance frequency, and said horn portion is configured to amplifydisplacement along said first axis.
 13. The ultrasonic horn of claim 12wherein said pre-stress value varies less than one part in 20 as saidfirst material deforms by one part in
 20. 14. The ultrasonic horn ofclaim 12 wherein said actuator material is a piezoelectric material. 15.The ultrasonic horn of claim 12 wherein said actuator material has a setof exterior dimensions and lacks a through hole.
 16. The ultrasonic hornof claim 12 wherein said first material comprises titanium.
 17. Theultrasonic horn of claim 12 wherein said resonance frequency is at leastfive thousand Hertz.
 18. The ultrasonic horn of claim 12 wherein saidultrasonic horn actuator is driven by a power of at least 20 Watts. 19.The ultrasonic horn of claim 12 wherein said ultrasonic horn actuator isattached to a support structure at a node of a resonant mode of theresonant frequency.
 20. The ultrasonic horn of claim 12 wherein saidresonance frequency is primarily determined by a length of said horn.